Errors arise in the recording of digital signals on a magnetic record carrier, such as a tape, which are usually due to mechanical discontinuities or faults on the surface of the recording medium, or due to changes in distance between a transducer, carried by a head wheel, in the recording apparatus from the surface of the magnetic medium, such as a tape. Such faults in the tape, or changes in distance between the recording head and the tape at incremental areas causes drop-outs and, consequently, loss of information in the particular area where the fault or change in distance occurred.
Magnetic storage of digitally coded video signals cause, frequently, at the point of drop-outs, false information regarding picture brightness. Scattered over the image which is reproduced, bright or dark portions of lines may occur which, due to their contrast, are annoying to the viewer with respect to the undistorted image content.
Various methods and systems have been proposed to cover errors and defects, and to effect correction, so that the effect of poorly recorded or non-recorded data are eliminated or, at least, ameliorated.
In one system, which has frequently been used, the stream of data which is received are recorded in a redundant recording system in order to be able to have a second, hopefully perfect recording available which may be resorted to. In another system, which also has been used, additional test words or bits are recorded in additional test tracks, from which the reproduction of defective data can be recognized, and the correct bits reconstructed.
A common disadvantage of the two above referred to methods and systems for correction of digitally coded data which are improperly recorded is that a substantial portion of the storage surface on the magnetic carrier is used to record redundant data or test data, respectively, and that this area thus is not available for the storage of the data which actually are to be reproduced. The storage density, thus, on the carrier is not an optimum.
Storage and transformation of video signals may also use a different type of method in order to improve reproduction in a magnetic transducer system using digitally coded data. This method is used primarily in the transmission of video signals. The above methods employ the characteristics of video signals that the video signal itself contains some redundancy. In a normal case, the information content of sequential video field lines is small; the difference between sequential frames likewise is small. In the circuits to carry out coverage of errors, the image content, after a time delay, provides a prognosis of the subsequent image content to be expected; if the actually received image content substantially deviates from the expected image content, then the preceding line, or at least a portion thereof, is repeated, and an average value is obtained by interpolation of the preceding line and the subsequent, apparently defective line, or portion thereof. This average value then replaces the data word or data blocks which are outside of the expected range of data. These systems and methods decrease the visible effects of errors in data in the transmission chain. The errors, themselves, are not eliminated and their consequences are not completely removed.